Recent seat belt retractors are provided with a pretensioner that drives a spindle to retract during a vehicle collision using gas pressure generated by exploding an explosive. Further, a retractor that includes a pair of pretensioners in order to increase a restraining force and a restraint amount of a seat belt without increasing the amount of explosive has been proposed (see Patent Document 1, for example). In this retractor, the pair of pretensioners, which are disposed on one side of a spindle, are activated simultaneously at the time of a vehicle collision and thereby joined to the spindle, whereupon the spindle is driven to retract.
Further, in a proposed retractor having a force limiter mechanism, in which energy is absorbed by causing a torsion bar to twist such that a seat belt is unreeled when a load of at least a set value acts on the seat belt during a vehicle collision, a pretensioner drives a spindle to retract via the torsion bar (see Patent Document 2, for example). According to this retractor, the restraining force generated by the pretensioner is transmitted to the spindle via the torsion bar, and therefore, when the spindle is rotated in an unreeling direction by activating the force limiter mechanism after driving the pretensioner, the operation of the pretensioner is not affected by this rotation.
Patent Document 1: Japanese Utility Model Laid-open No. H6-6131
Patent Document 2: Japanese Patent Application Laid-open No. H10-1025
Incidentally, in the seat belt retractor described in Patent Document 1, no mention is made of a force limiter mechanism. Therefore, in a case where this seat belt retractor is used together with a force limiter mechanism, gas pressure remaining in the pair of pretensioners may affect an operation of the force limiter mechanism when the force limiter mechanism is activated after the pretensioners drive the spindle to retract.
Further, in the seat belt retractor described in Patent Document 2, when an energy absorption load is smaller than a retraction load generated by the pretensioner, the torsion bar is twisted by the retraction load of the pretensioner, and as a result, the belt cannot be retracted sufficiently.